In an analog signal processing circuit including an analog/digital converter, in order to maximize the performance of the analog/digital converter, an analog signal inputted to the analog signal processing circuit is subjected to amplitude adjustment, and thereafter, input to the analog/digital converter.
As an amplitude adjustment function, a high-performance operating amplifier having a gain varying function has conventionally been adopted. The gain-variable operating amplifier is connected to a front stage of the analog/digital converter, and adjusts the conversion result of the analog/digital converter, or the amplitude of the analog input signal according to need, thereby performing signal processing (e.g., refer to Patent Document 1: Japanese Published Patent Application No. 2001-275012 (Page 5, FIG. 1)).
FIG. 7 is a diagram illustrating the construction of a signal processing circuit including the conventional analog/digital converter having the gain varying function.
With reference to FIG. 7, reference numeral 701 denotes a gain-variable operating amplifier, 702 denotes an analog/digital conversion circuit unit, 703 and 704 denote analog input signal inputting unit, 705 denotes a storage unit, and 706 denotes a judgment unit. The storage unit 705 holds the conversion result of the analog/digital converter 702, and sends a signal to the judgment part 706. The judgment unit 706 judges information such as the amplitude of the analog signal inputted to the analog/digital conversion circuit unit 702, and outputs a signal for performing gain adjustment of the gain-variable operating amplifier 701.
That is, in the construction of the signal processing circuit shown in FIG. 7, the analog signal to be input to the analog/digital conversion circuit unit 702 is amplified by the gain-variable operating amplifier 701, whereby the gain of the analog/digital converter is made variable in the entire circuit.